1. Field of the Invention
The present invention relates to a combination crane that integrates a plurality of crawler cranes in order to lift heavy materials.
2. Description of Related Art
Conventionally, in order to lift heavy materials, a so-called “multi-crane lift” which uses two crawler cranes is usually used. However, in said multi-crane lift, as load distribution to the respective cranes in operation changes in accordance with their operation postures, it is conventional to use two crawler cranes whose lifting capacities are enough to lift 100% of the working load respectively. Accordingly, when a lifting operation needing the double lifting capacity of the respective cranes is conducted, it is necessary to prepare two crawler cranes with the double lifting capacity. In this case, it is necessary to use a plurality of cranes so that a larger working space is needed and running cost becomes higher.
On the other hand, as a capacity increasing device which is available to increase its rated lifting capacity to one class higher than its original capacity, some devices are known, for example, a device capable of exchanging its booms, reinforcing its mast or connecting to a counterweight dory. However, such devices need heavier booms for replacement or an additional counterweight dory. Besides, in this case, crane work can not be performed for a load more than that of a single unit, because the crane itself is a single crane.
Therefore, in order to improve the efficiency of the crane operation without wasted working space and additional running cost, a self-propelling type combination crane is demanded, which works independently under normal working condition and also functions as a combination crane when it lifts a heavy material that a single crane can not handle. As a combination crane which fulfills this requirement, the crane shown in Japanese Patent Laid Open No. 2006-315864 (U.S. patent publication 2006/0273057) is known for example. This combination crane connects lattice boom members of two lattice boom cranes that are closely positioned in parallel. The mechanisms for tilting their lattice booms are mechanically or electrically operated in a synchronized timing in order to synchronously tilt two lattice booms.
However, as this combination crane is firmly connected by means of the lattice connection member, the direction of the main body and lattice boom of each crane can not change. This characteristic renders this conventional combination crane problematic when it is put into practical service.